1. Field of the Invention
This invention relates to adhesive compositions and more particularly to such compositions of a thermoplastic elastomer type suitable for use as pressure-sensitive adhesives.
2. Prior Art
Prior art pressure-sensitive adhesives are known which comprise elastomeric base polymers, resinous tackifiers, softeners and additives including anti-aging agents, antioxidants, fillers, colorants and the like. Such adhesives have found wide application to tapes, sheets and labels as well as carpets, hooks, thermal insulators, albums and the like. They are applied in a solution, hot-melt or emulsion form onto substrates such as cellophane, papers, plastic films, metallic foils and the like.
Elastomeric base polymers known in common use include natural rubber, reclaimed rubber, butadiene-styrene rubber, acrylonitrile-butadiene rubber, styrene-isoprene rubber, polyisoprene rubber, polybutadiene rubber, polyisobutylene rubber, chloroprene rubber, ethylene-propylene rubber, ethylene-vinyl acetate copolymer rubber and acrylic rubber.
Known tackifiers are rosins such as natural rosin, hydrogenated rosin, partially hydrogenated rosin and various rosin esters, terpene resins such as polymers of .alpha.-pinene, .beta.-pinene and dipentene, aromatic and aliphatic petroleum resins and their hydrogenates, cumarone-indene resin, terpene phenol resin and similar resins.
Materials commonly used as softeners are naphthenic mineral oils, polybutene, chlorinated paraffin, chlorinated biphenol, chlorinated triphenol, phthalate esters such as dioctyl phthalate, and tricresyl phosphate.
Pressure-sensitive adhesives after being blended if necessary with suitable additives are dissolved in an organic solvent, hot-melted or emulsified for use as aqueous emulsions, coated onto substrates and subsequently dried or cooled. Most widely employed is the solution coating which literally involves the use of large amounts of organic solvents. There have arisen serious problems associated with adverse work environments, air pollution and fire hazards due to such solvents being highly volatile and readily flammable.
An emulsion coating is known for its high speed performance and considerable labor saving, but is not satisfactory with respect to the foregoing problems. A keen demand has therefore been voiced for the availability of an improved pressure-sensitive adhesive composition particularly suitable for hot-melt coating.
Certain thermoplastic elastomers have been proposed as base polymers for hot-melt coating, but their combination with resinous tackifiers alone still has much to be desired for adequacy of tackiness and viscosity and hence need further blending with suitable softeners.
As such softeners, there may be used petroleum oils of relatively low aromatic hydrocarbon content, i.e. naphthenic and paraffinic oils, as disclosed for instance in Japanese Patent Publication No. 44-17037 since aromatic hydrocarbons would lead to reduced cohesion. The prior art petroleum oils however have a drawback in that they are not well compatible with tackifying resins, and furthermore, not satisfactory in regard to color, odor and acid resistance.